Inexpensive, and little setup, with no steep learning curve?

Off the top of my head:

• singing
• drawing
• playing a tin whistle
• dorodango
• dexterity games (card flourishes, astrojax, yoyo, pen spinning, etc.)
• reading
• kumihimo
• customising things with paint markers
• yoga
• whetstoning
• writing (prose or poetry)
• journaling/journalism
• cross stitch/embroidery
• jelqing
• rock/object balancing
• meditation
• knife throwing
• macramé (using beading floss)
• playing the spoons
• whittling
• eating
• scrimshaw
• nålbinding
• henna tattooing
• primping
• cloudgazing
• looking at fucking everything through a high mag loupe
• tarot divination
• contact juggling
• kefir making
• key duplication by hand
• sleeping
• hiking
• floccinaucinihilipilification
• origami
• talking to nearby friendly looking strangers
• polishing
• cleaning
• caligraphy
• drugless hallucination
• soapbox public speaking
• zen gardening
• guerilla gardening
• pitch ear training
• ‘easter egg’ hiding
• local botanical/ecological studies
• flint knapping
• home/interior design (as long as you don’t actually buy anything)
• knolling
• marble running
• solo RPGs like ‘Alone Among The Stars’ (okay this one wasn’t technically off the top of my head. I couldn’t remember the name, so I looked it up.)

That’s very little to go on, so my suggestively is reading.
Very cheap to get into as well as something that’s is easy to pickup and let rest. Plenty of free options around too.

Explore your neighborhood

• geocaching
• contributing to open street maps (Street complete)
• photography with your cell phone

There are exactly zero boring hobbies, through the right eyes or right lens.

boring as you use it, is a term which works best when describing a person. Take from that what you will.

Electric drum kit is fun, cathartic, and quiet outside your headphones. Chilis are easy to grow, and they certainly entertain the mouth. Running saved my life. Tennis balls from the machine launcher. Rock wall. Skateboard. People love pickleball, but you need friends. Darts. 3D printer. Electric piano. Cooking class you can meet a girl, cook for her, get back rubs. Singing lessons. Fetish exploration. You got this.

While you’re against boring, have you considered whittling?

Zero cost:

• Running
• Yoga
• Bodyweight exercise

Zero cost if you have a library card:

• Reading (fiction)
• Reading (non-fiction)
• Online discussion

Price of pencil and paper:

• Writing (creative)
• Writing (journaling)
• Drawing

Fairly cheap:

• Music (playing)
• Music (listening)
• Painting

Bird-watching or star-gazing or both. A pair of binoculars is all you really need for either. You can print some stuff off online if you wanna get fancier with it.

Origami. Very cheap to begin with, still cheap when you’re deep into it. Unless you got so deep that you’re travelling the world to go to conventions but that’s another story, the biggest expense is buying books once you decide to have them physically.

Don’t use printer paper, kraft paper is cheap and works perfectly. Only down side is having to cut it.

Cycling: Get a bicycle and start exploring your area. It doesn’t need to be a nice bike; check garage sales and Craigslist and get whatever fits comfortably. Wander off your usual routes and you will encounter new corners of your town that you never knew existed.

Music: Pick up a used acoustic guitar, electric keyboard, banjo, or whatever you can find. The world is full of inexpensive books and free YouTube videos to teach new students.

Drawing: Make doodles, little comics, or sketches of whatever is around you. A pencil and paper are all you need. Look up YouTube videos for daily sketching practice exercises.

Amateur radio: An entry-level license involves a basic quiz and a nominal fee. Cheap radios can be had for less than US$100. If you meet an old timer with a garage full of gear they might hand you a pile of old equipment for free.

Of course, any of these can become expensive and time-consuming hobbies if you go down the rabbit hole. But that just means you have found something you love.

Painting. Can be done for relatively cheap and scales up to a lot.

Birdwatching. If you have a smartphone you can get the merlin app and start from there. Collect 'em all like pokemon.

Photography. Really helps you see everything with a new perspective. Learn natural scapes, urban scapes. Interesting people. Flowers, Social activism documenting homeless, or cool or interesting people if you live in an urban freakshow. You can make the ordinary extraordinary.

May I suggest simple crafting, like whittling, ancient/native pottery, wood carving, paper basket weaving, origami, crochet (or the slightly more addictive and expensive knitting), braiding, weaving, macrame, yarn spinning, decorative knotworking, flower pressing, dyemaking, hand dying, flower arrangement, make walking canes, etc?

Or maybe an instrument? Someone mentioned guitar, but why not harmonica, ukulele, bongos, simple percussives, or why not take up singing, there’s typically a choir almost everywhere, or you could try solo, or whistling, or yodeling.

Or maybe take up gardening/growing? In pots, hydroponically, maybe on a growing lot, or someone’s yard?

Or take up community projects, like either sanctioned or guerilla gardening, greening, forestry, litter picking, maintaining communal spaces, volunteer, start a community project (pollination diversity, bee keeping, growing lots, food drive, food forestry)

Or take up a cause? Build community, take affirmative action, demonstrate, protest, coordinate, communicate, with an established org, or on your own?

Maybe the outdoors could entice? Take up walking, ornithology, watch bats, discover new insects, take up chabana/ikebana, learn to forage, learn survival skills, go hiking, stack rocks, track animals, learn trapping, get into scouting, make bark boats, learn to skip rocks, climb trees, jump in puddles, pretend (or practice) animistic rituals, feel rain on naked skin, sleep under a bare sky, pick skulls, horns and bones to craft with.

Perhaps create stories, tell stories, write poetry, learn magic, juggle, poi, take up clowning, go swing/country/folk/traditional/ballroom/social dancing, try Butoh, write sketches, join a theatre, start a business, take up decorating, or just embrace whimsy?

I can also warmly recommend trying a new thing every week/month/period and see what sticks and not.

Needle felting. Like ten bucks for some needles, some wool, and a sponge. It’s a no-skill-required art form that consists entirely of stabbing.

I just want to point out that any hobby you engage in for entertainment without it feeling like a chore is by definition not boring (to you). Personally I couldn’t see myself in a situation where I’d be collecting stamps, because I would probably find it very boring. But others enjoy it, so it’s a valid un-boring hobby, just not for me.

So here’s the problem: you can’t adopt someone else’s hobby, and find it interesting by proxy. Any activity (not just hobbies) can only be “not boring” if you yourself are actively interested in it for its own sake, not just as a time sink. Your attention and investment in the activity is what makes it interesting and engaging. You get out of it what you put into it. Low-energy, low-investment time sinks are entertainments, not hobbies. Play some video games, no shame in that, it’s relaxing, inexpensive, easy to start and you can pick it up and put it down whenever.

To really give you an answer on a hobby, we need to know about what you’re interested in. Hobbies are about developing some kind of skill, for your own benefit and growth as a person (probably not financial benefit). Hobbies are about exercising your personal passions on your own terms, with no obligations except those which you set for yourself. What kinds of skills do you have as a starting point, and what do you wish you had? What kinds of activities do you enjoy doing? What kinds of activities do you actually make time in your life to do, on your own, regardless of whether other people are involved?